A family of proteins of diverse functions in the development of the nervous system. In mammals, they are products of 4 genes, with proteins coded by nrg1 being the most diverse. It influences upregulation of acetylcholine receptor genes beneath the endplate (in this instance known as ARIA = Acetylcholine Receptor Inducing Activity). In development of synapses at the neuromuscular junction, after mammalian motor neurones have made synaptic contact with muscle fibres.